“不客气".NET 中的实例方法_C#/.NET开发问题-跟版网

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问题描述

您能否在创建时不指定实例的情况下创建实例方法的委托?换句话说，您能否创建一个静态"委托，将调用该方法的实例作为其第一个参数?

Can you create a delegate of an instance method without specifying the instance at creation time? In other words, can you create a "static" delegate that takes as it's first parameter the instance the method should be called on?

例如，如何使用反射构造以下委托?

For example, how can I construct the following delegate using reflection?

Func<int, string> = i=>i.ToString();

我知道我可以使用 methodInfo.Invoke，但这会比较慢，并且在调用它之前不会检查类型正确性.

I'm aware of the fact that I can use methodInfo.Invoke, but this is slower, and does not check for type-correctness until it is called.

当您拥有特定静态方法的MethodInfo时，可以使用Delegate.CreateDelegate(delegateType, methodInfo)，静态方法的所有参数保持自由.


When you have the MethodInfo of a particular static method, it is possible to construct a delegate using Delegate.CreateDelegate(delegateType, methodInfo), and all parameters of the static method remain free.
正如 Jon Skeet 所指出的，如果方法在引用类型上是非虚拟的，您可以简单地应用相同的方法来创建实例方法的开放委托.决定在虚拟方法上调用哪个方法很棘手，所以这不是那么简单，而且值类型看起来根本不起作用.
As Jon Skeet pointed out, you can simply apply the same to make an open delegate of an instance method if the method is non-virtual on a reference type.  Deciding which method to call on a virtual method is tricky, so that's no so trivial, and value-types look like they don't work at all.
对于值类型，CreateDelegate 表现出非常奇怪的行为:
For value types, CreateDelegate exhibits really weird behavior:
var func37 = (Func<CultureInfo,string>)(37.ToString);
var toStringMethod = typeof(int).GetMethod("ToString", BindingFlags.Instance | BindingFlags.Public, null, new Type[] {typeof(CultureInfo) }, null);
var func42 = (Func<CultureInfo,string>)Delegate.CreateDelegate(typeof(Func<CultureInfo,string>), 42, toStringMethod,true);
Console.WriteLine( object.ReferenceEquals(func37.Method,func42.Method)); //true
Console.WriteLine(func37.Target);//37
Console.WriteLine(func42.Target);//42
Console.WriteLine(func37(CultureInfo.InvariantCulture));//37
Console.WriteLine(func42(CultureInfo.InvariantCulture));//-201040128... WTF?

如果实例方法属于值类型(这适用于引用类型)，则以 null 作为目标对象调用 CreateDelegate 会引发绑定异常.
Calling CreateDelegate with null as the target object throws a binding exception if the instance method belonged to a value type (this works for reference types).
几年后的一些跟进:导致func42(CultureInfo.InvariantCulture);返回"-201040128" 在我的示例中，而不是 "42" 是内存损坏，可能允许远程执行代码 (cve-2010-1898);这已在 2010 年的 ms10-060 安全更新.当前框架正确打印 42！这并没有使回答这个问题变得更容易，但解释了示例中特别奇怪的行为.

Some follow-up years later: The incorrectly-bound target that caused func42(CultureInfo.InvariantCulture); to return "-201040128" instead of "42" in my example was memory corruption that could have allowed remote code execution (cve-2010-1898); this was fixed in 2010 in the ms10-060 security update.  Current frameworks correctly print 42!  That doesn't make answering this question any easier, but explains the particularly weird behavior in the example.
推荐答案
您实际上选择了一个特别棘手的示例，原因有两个:
You've actually chosen a particularly tricky example, for two reasons:
ToString() 是继承自 object 但在 Int32 中被覆盖的虚方法.
int 是一个值类型，当涉及到值类型和实例方法时，Delegate.CreateDelegate() 有一些奇怪的规则——基本上第一个有效参数变成了ref int 而不是 int


ToString() is a virtual method inherited from object but overridden in Int32.
int is a value type, and there are weird rules with Delegate.CreateDelegate() when it comes to value types and instance methods - basically the first effective parameter becomes ref int rather than int

但是，下面是 String.ToUpper 的示例，它不存在这些问题:
However, here's an example for String.ToUpper, which doesn't have either of those problems:
using System;
using System.Reflection;

class Test
{
    static void Main()
    {
        MethodInfo method = typeof(string).GetMethod
            ("ToUpper", BindingFlags.Instance | BindingFlags.Public,
             null, new Type[]{}, null);

        Func<string, string> func = (Func<string, string>)
            Delegate.CreateDelegate(typeof(Func<string, string>),
                                    null,
                                    method);

        string x = func("hello");

        Console.WriteLine(x);
    }
}

如果这对你来说足够好，那太好了......如果你真的想要 int.ToString，我将不得不更加努力:)
If that's good enough for you, great... if you really want int.ToString, I'll have to try a bit harder :)
这是一个值类型的例子，使用一个新的委托类型，它通过引用获取它的第一个参数:
Here's an example for a value type, using a new delegate type which takes its first parameter by reference:
using System;
using System.Reflection;

public struct Foo
{
    readonly string value;

    public Foo(string value)
    {
        this.value = value;
    }

    public string DemoMethod()
    {
        return value;
    }
}

class Test
{
    delegate TResult RefFunc<TArg, TResult>(ref TArg arg);

    static void Main()
    {
        MethodInfo method = typeof(Foo).GetMethod
            ("DemoMethod", BindingFlags.Instance | BindingFlags.Public,
             null, new Type[]{}, null);
        RefFunc<Foo, string> func = (RefFunc<Foo, string>)
            Delegate.CreateDelegate(typeof(RefFunc<Foo, string>),
                                    null,
                                    method);

        Foo y = new Foo("hello");
        string x = func(ref y);

        Console.WriteLine(x);
    }
}


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